The present invention relates to the correction of printing errors caused by printhead misalignments in inkjet printers and plotters and in particular to misalignments due to relative rotation between a printhead and the print media to be printed on.
An ink jet printer is a non-impact printing device that forms characters and other images by ejecting ink drops in a controllable way from a printhead. Ink jet printing mechanisms may be used in different devices such as printers, plotters, facsimile machines, copiers and the like. For the sake of convenience, in what follows reference will be made only to large format ink jet printers or plotters to illustrate the concepts of the present invention.
The printhead of a machine of the kind mentioned ejects ink through multiple nozzles in the form of minuscule drops which xe2x80x9cflyxe2x80x9d for a small space and strike a print media. Different printheads are used for different colors. Ink jet printers usually print within a range of 180 to 2400 or more dots per inch. The ink drops are dried upon the print media soon after being deposited to form the desired printed images.
There are several types of ink jet printheads including, for example, thermal printheads and piezoelectric printheads. By way of example, in a thermal ink jet printhead the ink drops are ejected from individual nozzles by localised heating. Each of the nozzles has a small heating element. An electric current is made to pass through the element to heat it. This causes a tiny volume of ink to be heated and vaporised instantaneously by the heating element. Upon being vaporised, the ink is ejected through the nozzle. An exciter circuit is connected to individual heating elements to supply the energy impulses and, in this manner, to deposit in a controlled way ink drops from associated individual nozzles. These exciter circuits respond to character generators and other imaging circuits to activate selected nozzles of the printhead in order to form the desired images on the printing support.
Thermal inkjet printing is based on accurate ballistic delivery of small ink droplets to exact locations onto the paper or other media. One key factor for sharp and high quality images stems from the accuracy of the droplet placement. Droplet placement inaccuracy results in fact in line discontinuity and roughness, as well as banding and colour inconsistencies.
Droplet placement inaccuracies are caused by imperfections and variations of the mechanical and geometrical characteristics of the printer and printhead, and the positioning of the printhead within a carriage of the printer as well as their functional performances. The defects caused by droplet placement errors appear in a variety of ways and may depend on the print modes being used (i.e. the sweep velocity of the printhead over the paper and the direction of printing).
Full colour printing and plotting requires techniques for correcting different causes of droplet placement inaccuracies. Some of these techniques, using a sensor module for measuring printing errors in appropriate printed patterns, are disclosed in EP 0 622 237.
One way to tackle this problem is to impose tight specifications on all sources of variations but for achieving a reasonable trade-off between quality and yield there is a need for correction methods for droplet placement errors.
EP 0 622 237 discloses systems for correcting some causes of droplet placement errors, in particular those which are due to printhead offsets in the scan and the media axis. These systems are currently implemented in printers/plotters as default printhead alignment procedures to be carried out in particular circumstances, i.e., change of printheads. None of these systems applies corrections for droplet placement errors caused by relative rotations between the printhead and the printing surface.
However, the trend of increasing print productivity, in particular in large format printers/plotters, by means of new printheads with more nozzles, makes those new printers more vulnerable to said errors.
According to a first aspect of the present invention there is provided a method, applicable to an inkjet printer having a scanning carriage, capable of bidirectional scanning along a scan axis, in which at least one printhead is mounted, for correcting for drop placement errors due to relative rotation between the printhead and the print media to be printed on, the method comprising the steps of: first determining the relative contribution to the drop placement error due to rotation of the printhead about the scan axis (Y axis error), then, with respect to any determined Y axis error, applying the same magnitude and sense of correction for drop placement errors while printing in both a first scanning direction of the carriage and while printing in a second scanning direction of the carriage. It is believed by the present applicants that no prior art inkjet printers correct for drop placement error due to rotation of the printhead about the scan axis.
Preferably, the method comprises printing by the printhead a test pattern on print media in which either Y errors or both Y and Z errors manifest themselves and measuring said test pattern to determined said errors.
According to a second aspect of the present invention, there is provided apparatus for correcting for drop placement errors in an inkjet printer due to relative rotations between the printhead and the print media to be printed on, comprising: a processor to store and apply correction parameters for the firing time of nozzles of said printhead wherein said stored correction parameters have been determined in accordance with the method of the present invention.
Preferably, the apparatus further comprising a test pattern generator for printing a test pattern on print media and a sensor module for obtaining measurements from said printed test pattern and the processor is capable of generating said correction parameters in dependence on the measurements made from said printed test pattern.
The droplet placement errors caused by rotations of the printhead around the carriage scan or Y axis and vertical or Z axis and translations of the printhead along the Z axis manifest themselves clearly when printing vertical lines on the print media since they appear rotated or broken into segments.
Rotations of the printhead around the Z axis cause correspondingly identical rotations of the printed vertical lines with respect to the ideal vertical direction on the paper or X axis. These errors are independent of the print direction and will be called unidirectional rotations or Z errors.
Rotations of the printhead around the Y axis cause proportional rotations of the printed vertical lines with respect to the ideal vertical direction on the paper (X axis), but these errors depend on the print direction and the carriage velocity among other factors. These errors will be called bidirectional rotations or Y errors.
Translations of the printhead along the Z axis (changes in the printhead to paper spacing), cause translations of sections of the printed vertical lines along the paper Y axis, dependant on the print direction and the carriage velocity among other factors, these will be called bidirectional translations or B errors.
All these errors occur without substantial variations along the length of the scan axis.
Although alternative techniques for determining these errors are contemplated (for example by mechanical measurements of the position of the printhead relative to the print media when the printhead is mounted in the carriage) according to preferred embodiments of the present invention, in the first step a test pattern is printed in which said errors manifest themselves.
In a second step, in said test pattern the errors of a unidirectional and bidirectional nature are measured with the sensor module. In a third step differentiated correction parameters are obtained for the errors consisting of bidirectional movements, bidirectional rotations and unidirectional rotations.
As the printer has the possibility to fire different nozzles with adjustable relative advances and/or delays, said parameters are used to modify the firing electronics. Thus a droplet which overshoots its ideal position can be ejected in advance, and a droplet which falls short of its ideal position can be fired with a delay, so as to deliver both to their exact location.
The correction method of the present invention can be included in the printhead alignment procedures incorporated into the printer/plotter to jointly correct the mentioned B, Y and Z errors. It can also be used to correct just some of the mentioned errors.